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Recommendations and Requirements Derived from Lessons Learned

All important organizations which are engaged in nuclear safety regulation have analyzed the Fukushima accident and have identified lessons learned and proposed recommendations which evolved from these lessons. These bodies were, for example, IAEA, NRC, ENSREG, ANS and Japanese organizations such as AESJ.

New regulatory requirements for commercial light water nuclear power plants were developed in Japan in July 2013, taking into account the lessons learned from the accident at Fukushima Daiichi Nuclear Power Station [3]. Major improvements include:

• Enforcement of resistance against earthquake and tsunami,

• Reliability of power supply,

• Measures to prevent core damage by postulating multiple failures,

• Measures to prevent failure of containment vessel,

• Measures to suppress radioactive material dispersion,

• Strengthen command communication and instrumentation,

• Consideration of natural phenomena in addition to earthquakes and tsunamis, for example volcanic eruptions, tornadoes and forest fires,

• Response to intentional aircraft crashes,

• Consideration of internal flooding, and

• Fire protection

These improvements are specifically required to be installed within the current Japanese reactor fleet as basic requirement for an allowance of further operation.

Examples for Potential Countermeasures and/or Technologies to be Applied

On basis of the identified lessons and countermeasures, some examples are described in more detail in the following sections of this chapter. There are three main areas selected as follows:

• External events,

• Design of buildings, systems and components, and

• Severe accident issues

External Events

There are some common countermeasures proposed for all external events which are considered to be generally applied for all extreme external events as follows:

• Develop an approach to regulate hazards from extreme natural phenomena;

• Periodically redefine and re-analyze the natural event design basis.

Since external events in most cases lead to a combination of initiating events such as earthquake and tsunami or earthquake and fire, such combined effects have to be systematically considered in the design. One proposal which could be considered as a good approach is recommended by Sustainable Nuclear Energy Technology Platform (SNETP) [4] as follows:

• Extending even further the in-depth safety approach to any type of hazards, in particular external ones, and accounting for any mode of combination of them;

• Systematically include the design extension conditions (beyond design basis

accidents) in the defense-in-depth approach at the design stage.

According to SNETP, there is also the need for future studies and development in the following area:

• Development of approaches to natural hazard definition, techniques and data, and development of guidance on natural hazards assessments, including earthquake, flooding and extreme weather conditions;

• Development of guidance on the assessment of margins beyond the design basis

and cliff-edge effects for extreme natural hazards;

• Development of a systematic approach to extreme weather challenges and a more consistent understanding of the possible design mitigation measures;

• Development of the approach for assessment of the secondary effects of natural

hazards, such as flood or fires arising as a result of seismic events;

• Enhancement of probabilistic safety analysis (PSA) for natural hazards other than seismic (in particular extreme weather) and development of methods to determine margins and identify potential plant improvements;

• Overall enhancement of PSA analysis, covering all plant states, external events

and prolonged processes, for PSA levels 1 and 2. Earthquake

It is proposed from several organizations to increase the seismic design criteria for the evaluation and assessment of beyond design external events. There are some proposals available, such as those from Ref. [5], to increase the seismic design criteria to 1 degree of magnitude e.g. 0.2–0.3 g. Yet, there is no final decision that can be commonly agreed upon within the nuclear community. This is one of the tasks that have to be worked on by the respective organizations in the future.

It is now common understanding that a periodically redefinition and re-analysis of the earthquake design basis should be performed in the future. The regulatory basis has to be provided by the respective organizations.

In Japan, Nuclear Regulatory Authority (NRA) strengthened the examination of active faults. Seismic design needs to take into account of the faults that was active after 126,000 years ago (Late Pleistocene). If necessary, activity of the faults is examined up to 400,000 years ago. The ground acceleration should be determined taking the three-dimensional underground structures, which may amplify the acceleration. The safety-class structures and buildings should not be built on the active faults. The ground acceleration increases with the length of active faults. The length of faults needs to be determined including the examination of nearby seabed. Big earthquakes such as the movement between continental plates also need to be considered separately. The basic earthquake ground motion is determined from these points. It changes with the site of the nuclear power plants. Strengthening the seismic design of the plants is conducted after the approval of NRA.

Fig. 12.2 Installation

of a seawall to prevent site inundation [3] Tsunami

The common countermeasures described above are also proposed for tsunami events.

As an example the standards set by the Japanese NRA define a “Design Basis Tsunami” as one that exceeds the largest ever recorded. It requires protective measures such as seawalls. The standards also require “structure, systems and components (SSCs)” for tsunami protective measures to be classified as class S, the highest seismic safety classification to ensure that they continue to prevent inundations even during earthquakes.

The examples of multi-layered protection measures against tsunami are installation of a seawall to prevent site inundation and installation of water-tight doors to prevent the flooding of buildings. An example for a seawall is shown in Fig. 12.2.

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